This invention relates to the preparation of fluoride glass, and, more particularly, to the preparation of fluoride glass by chemical vapor deposition.
As commonly used, the term "glass" refers to materials that are transparent to radiation such as visible light, so that they permit radiation energy to pass or conduct the radiation, but prevent passage of matter. The radiation is ordinarily thought of as being visible light, but can also include those forms of radiation that are not visible to the human eye. For example, infrared energy, having a wavelength greater than that of visible light, is not visible to the human eye. Infrared light includes electromagnetic radiation having wavelengths of from about 0.8 to about 8 micrometers, and sometimes beyond. Infrared light is used in a variety of devices, including fiber optic communications systems, detectors, photocells, vidicons, and the like. It is therefore important to have glasses that are good optical conductors for use with infrared radiation.
Windows for visible light, such as those commonly found in the home, are made of silicon dioxide based glasses. These glasses are readily prepared and are highly transmissive to visible light having wavelengths of from about 0.3 to about 0.7 micrometers, and to certain other forms of electromagnetic radiation. However, the silicon dioxide glasses have much poorer transmission of infrared energy, and generally cannot be used as windows for infrared energy.
The metal fluoride glasses are known to have good transmission to infrared radiation, and have been successfully tested for use in infrared systems. The production and use of metal fluoride glass pose some difficult challenges. Special care must be taken during production of the glass to prevent harmful or dangerous fluoride reactions, and to avoid degradation of the glass in use. Metal fluoride glasses are conventionally prepared by melting a mix of the necessary fluoride components, and quenching the melt to a supercooled state to form the glass. Precautions are taken to avoid undersired reactions.
Glasses to be used as the conductor portion of optical waveguides and related devices are often prepared by depositing the chemical mixture of the glass on the inside of a glass tube. The deposited material is converted to the vitreous state, and then the tube is drawn to a fine filament, so that the deposited glass becomes a light-conducting core that is enclosed within the outer glass case of the material originally forming the glass tube. The casing confines transmitted light to the central core as a result of the difference in the values of the refractive indices of the casing and the core.
This technology is well established for silicon dioxide and similar types of core glass used in conducting visible light. In one approach, called chemical vapor deposition or CVD, two or more selected gases containing the volatilized individual ingredients of the glass are passed into the interior of a heated glass outer tube, with the result that the gases react to deposit the glass-forming ingredients on the inside surface of the tube, in a form known as glass soot. The glass tube and glass soot are heated to melt the soot, further heated to collapse the tube, and cooled to convert the glass soot to the vitreous state. This collapsed tube, termed a preform, is then drawn to a fine size using glass drawing techniques.
The result of this process is a composite glass structure, having a continuous central glass optical conductor or waveguide that conducts the light, lying along the center axis of an outer protective glass covering. Chemical vapor deposition provides an excellent method for forming such glass composites with one type of glass in the center of a tube of another type of glass.
The key to using chemical vapor deposition is to find reactant compounds of the required glass forming elements that can be made sufficiently volatile, and which do not leave unintended residues mixed with the deposited glassy materials. Such compounds and techniques are well established for oxide glasses such as silicon dioxide glasses. No practical combination of compounds and techniques is as yet known for depositing metal fluoride glasses, so that chemical vapor deposition of metal fluoride glasses to be used in infrared waveguides, for example, is not practiced commercially.
There is therefore a need for a chemical vapor deposition process for depositing metal fluoride glasses. Such a process should be operable to produce such glasses, be economically and commercially practical, and not involve excessively hazardous or dangerous reactant gases that cannot readily be handled. The present invention fulfills this need, and further provides related advantages.